This invention relates to a semiconductor laser device which can emit laser beams possessing plural different oscillation wavelengths.
In the field of recent optical information processing, optical recording and reproducing apparatus such as optical disks have come to be used. In such apparatus, semiconductor lasers for writing, reading and erasing the data are used. Depending on the application, it may be desired to read out right after writing the data, or to write while erasing and then read. In this case, it is advantageous when the wavelength of the semiconductor laser beam for writing (.lambda..sub.W) and that of the semiconductor laser beam for reading (.lambda..sub.R) are different (.lambda..sub.W &gt;.lambda..sub.R). This is because, since these semiconductor lasers are disposed close to each other, mixing in of the writing signal while reading must be avoided, and the spot diameter of the laser beam for reading must be reduced (to shorten the wavelength) for the purpose of accuracy of the reading signal.
Besides, when recording a high quality television picture, too, it is desired to write the brightness signal and color signal by two laser beams differing in wavelength. In such situation, lately, there is an increasing demand for assembling a plurality of semiconductor lasers differing in wavelength into one integral chip. Meanwhile, a semiconductor laser device to emit plural laser beams is strongly demanded also as the light source of optically multiple communications of large capacity.
Conventionally, as a one-chip semiconductor laser capable of emitting plural laser beams, a structure of laminating an ordinary double hetero structure in two layers, removing part of upper double hetero structure, and forming an electrode for a semiconductor laser corresponding to the lower double hereto structure is known [Shiro Sakai; Electronics Lett, 18 (1982) 17]. This semiconductor laser has two semiconductor laser parts, in which when one semiconductor laser part is driven, a laser beam of oscillation wavelength .lambda..sub.1 is emitted, and when the other one is driven, a laser beam of oscillation wavelength .lambda..sub.2 is emitted (.lambda..sub.1 .noteq..lambda..sub.2).
Since the electrode material of one semiconductor laser (e.g., Au/Zn) is different from that of the other (e.g., Au/Sn), at least three electrode forming processes are needed. The process is complicated also because the active region of each semiconductor laser is composed of different epitaxial layers. Furthermore, when the gap between the two electrodes provided in the upper double hetero structure is regarded as one semiconductor laser, the sheet resistance increases in the region beneath the electrode located at the lower position, so that the threshold current of oscillation is raised as compared with that of the semiconductor laser between the lower electrode and said electrode at the lower position.